In egg-laying hens, medullary bone serves as a reservoir of labile calcium to be used during egg shell calcification. The purpose of this study is to characterize medullary bone cell changes during the egg-laying cycle and investigate the factors which might regulate and control these events. Medullary bone osteoclasts appear to be modulating their activity but not their population during the 24 h egg cycle of the Japanese Quail. Osteoclasts are uniformly synchronized, actively resorbing bone during egg-chell calcification and are 'inactive' and lack cell surface ruffled borders during the remainder of the cycle. This uniform synchronization of cells offers a unique system to study the mechanisms of activation and inactivation of bone cells, particularly the osteoclast. It is also of interest to define the factors responsible for the regulation of calcium metabolism and bone cell changes in hens. Initial studies will center on the possible role of estrogens, androgens, vitamin D metabolites, calcitonin and possibly pituitary factors.